Conventional thermal protectors include a protector having three pairs of contacts as disclosed in JP-B-46-34532 and a protector having two pairs of contacts as disclosed in JP-A-1-105435 and JP-A-10-21808.
The number of movable and fixed contacts is six in the thermal protector with the three pairs of contacts, which number is non-economical. Further, the three movable contacts are secured to a metal plate serving as a heating resistor, and the metal plate is supported in its central portion by a thermally responsive plate. The central portion of the metal plate is pressed such that the three movable contacts are uniformly pressed, whereupon a stable contacting is achieved. However, the metal plate fixed by caulking or the like in a through hole provided in the central portion of the thermally responsive plate drawn into the shape of a dish. In short, the metal plate is supported on the central portion of the thermally responsive plate, on which portion stress concentrates. Accordingly, stress applied to the thermally responsive plate differs depending upon a degree at which the metal plate is caulked relative to the thermally responsive plate, whereupon the characteristic of the thermal protector tends to easily change. That is, there arises a problem that it becomes difficult to stabilize the performance of the thermal protector.
On the other hand, a movable contact is secured to the thermally responsive plate itself in the thermal protector having the two pairs of contacts. Electric current is caused to flow through the thermally responsive plate so that its heat generation reverse the thermally responsive plate to open the contacts. This type of thermal protector is called direct heat type. Since the thermally responsive plate is heated up by the electric current in the thermal protector of the direct heat type, a response speed of the thermally responsive plate to an overcurrent is advantageously increased.
However, since a part which generates heat is limited to the thermally responsive plate, the peripheral components is difficult to heat up. Accordingly, when the thermal protector operates such that a current path is cut off, heat generated by the thermally responsive plate is absorbed by the peripheral components whose temperatures are relatively lower, whereupon a contact opening time cannot be rendered longer. As a result, the temperature of a motor winding having been increased by the overcurrent cannot be reduced sufficiently during cutoff of current such that a temperature reached by the motor winding is inevitably rendered higher while the thermal protector repeats its reverse and return. In this case, there is a problem that the increased temperature reduces the insulating performance of an insulating coating of the motor winding thereby to cause a short circuit which leads to possible burn-out.
Further, when a bimetal or trimetal each with a suitable curvature and operating temperature is selected as a material for the thermally responsive plate, the specific resistance of the thermally responsive plate does not always take a suitable value. That is, there is a problem that it is difficult to design a thermal protector having both suitable values of operating current and operating temperature.
The applicant invented a thermal protector which overcame the foregoing problems and filed a patent application for the invention in Japan (laid open under JP-A-2000-229795). This thermal protector is of an indirect heat type in which a thermally responsive plate is reversed by heat generation of a heating resistor. In this protector, the temperature of the thermally responsive plate is increased by heat radiation from the heating resistor when the current increases the temperature of the heating resistor. When an overcurrent or the like excessively increases the temperature of the heating resistor such that the thermally responsive plate reaches a set operating temperature, the thermally responsive plate quickly reverses thereby to cut off the current path. Not only the temperature of the thermally responsive plate but also the temperatures of peripheral components are increased by the heating resistor in the thermal protector of the indirect heat type. Accordingly, since heat is difficult to be absorbed from the thermally responsive plate to the periphery, it takes more time for the temperature of the thermally responsive plate to decrease. As a result, it takes more time for the temperature of the thermally responsive plate to decrease, whereupon the contact opening period of time can be rendered longer. Thus, since the temperature of the motor winding is sufficiently decreased during the contact opening period of time, the winding can reliably be protected against burnout. Further, the design of the thermally responsive plate can easily be carried out since the thermally responsive plate can be designed only in consideration of the reversing temperature.
However, when a protector is arranged which has a large operating current exceeding 200 A, there arises a defect that a large current also flows through components on the current path other than the heating resistor. For example, a large current also flows through an elastic member supporting the heating resistor in the above-mentioned thermal protector. As a result, the elastic member itself is heated more or less. When the elastic member is repeatedly heated for a long period of time, the elastic member looses its elasticity, whereupon the contacts cannot be opened. As a countermeasure for this problem, a thickness of the elastic member is increased so that a resistance value thereof is decreased thereby to reduce an amount of heat generated. However, the thickness of the elastic member cannot be increased over the value allowing elastic deformation. This results in an upper limit of the operating current of the thermal protector, whereby a thermal protector having a large operating current cannot be arranged.
Therefore, an object of the present invention is to provide a thermal protector which can be coped with a large operating current in the arrangement that the thermally responsive plate is revered in response to the heating of the heating resistor thereby to cut off the current path.